1. Field of the Inventions
The present invention relates to a metal fastener with a partial lubricating coating. The technical field of the invention is generally that of fasteners. More specifically, the invention relates to screws, lock groove fasteners, and any other metal fastener intended to assemble structural elements, such as pre-bored elements of an aircraft. Such fasteners are generally made from a metal material like stainless steel or a titanium alloy.
2. Related Art
The new generation of airplanes uses a composite material structure with the benefit of significantly lessening the mass of the airplane and not being sensitive to the phenomenon of fatigue. However, unlike a metal structure, composite materials have the drawback of poorly conducting electricity, and pose problems of lighting strike resistance. In order to resist lightning, it is necessary to close up all spaces that may exist between the fastener and the bore in order to prevent the electrical breakdown of the air.
In the prior art, document WO 2011/050040 teaches sleeve fasteners installed with an interference fit in composite materials. An interference fit is defined by installing a fastener whose outer diameter is greater than the bore diameter that will receive it, which causes the bore to expand when installing the fastener. The fastener comprises a rod comprising a lubricating coating on the threaded portion and on the transitional portion between the straight shank and the threaded part, to facilitate the insertion of the rod into the sleeve. The drawback of this fastening system is that it includes very little lubricating coating on the shank, which may require very great force to install the fastener.
In addition, the thicker the structures to assemble are, the greater the force required to insert an interference fastener. They may exceed the maximum force for installing the fastener above which the fastener breaks, whether in the threads, or in a traction groove, or in the installation tool.
It is also known that document GB 2 212 580 teaches an unsleeved metal fastener installed in a composite material structure. This fastener comprises a dielectric lubricating coating—of the TEFLON™ or MoS2 type—on a portion of its shank, allowing the fastener to be inserted into the bore. The portion of the shank connected to the head, supported by an outer surface of the aircraft's structure, which may be struck by lightning, is left bare in order to allow electrical currents to pass from the fastener's head into the structure of the aircraft via the bare metal shank portion, which is conductive. The drawback of this fastener is that it has a single conductive portion placed arbitrarily close to the head, which requires that the adjacent structure be relatively conductive, or comprise a conductive metal mesh.